1. Field of the Invention
This invention relates generally to fuel cells and more particularly to fuel cell systems that can be used in sulfur environments.
2. Description of the Related Art
Fuel cells liberate electrochemical energy from fuel streams containing hydrogen and/or other gases. A particular type of fuel cell, known as a solid oxide fuel cell (SOFC), has the ability to produce energy from hydrocarbon fuels at efficiencies far greater than traditional combustion engines, potentially as high as 80% for integrated systems. The discovery of a feasible energy production process is very important, given that natural gas and oil reserves are at low levels and continue to diminish. Though coal is also a limited resource, very large quantities still exist in many countries, including the USA. Coal can be used as a fuel for SOFCs if it is gasified to form a fuel known as “coal syngas”.
A diagram demonstrating how a typical SOFC works is shown in FIG. 1. Another simple diagram of a basic SOFC is shown in FIG. 2. Since SOFC are electrochemical devices, they consist of three main components: an anode, a cathode and an electrolyte. As shown in FIG. 1, available CO and H2 in the fuel stream are utilized at the SOFC anode. H2 oxidizes more readily than CO due to the faster diffusion rate of H2 into the porous anode. The fuel stream reacts in the triple phase boundary (the area where the fuel, oxygen ions, and electrons produced by the oxidation are present) of the anode. The electrons produced by oxidation constitute the electrical power produced by the fuel cell. After reaching a power load, the electrons travel to the cathode of the SOFC where oxygen from air is reduced to oxygen ions (O2−). The oxygen ions then travel across an electrolyte, such as yttria stabilized zirconia (YSZ), that only allows the passage of oxygen ions. The ions then complete the circuit when they reach the anode.
During operation, a fuel stream containing H2 and/or CO flows over the anode, while the cathode is exposed to either oxygen or air. When a load is applied to the system, oxygen reduces at the cathode to form oxide ions as noted above and according to the following:O2+4e−→2O2−  (1)These ions migrate through the electrolyte to the anode, where they react with the fuel stream components to produce an electrical charge according to the following:2CO+2O2−→2CO2+3e−  (2)2H2+2O2−→2H2O+4e−  (3)
H2S is a colorless, poisonous gas that is present in gasified coal and can cause many problems throughout SOFC systems, most notably to the anode. The SOFC shown in FIG. 1 shows little to no resistance in H2S-containing environments. The activity of a typical SOFC anode drops considerably after exposure to H2S concentrations as small as 2 ppm. In the presence of larger concentrations of H2S, this effect can be irreversible.
Therefore, in order to use gasified coal as a fuel source for SOFCs, either the anodes in the SOFC must be tolerant to H2S, or there must be no H2S present in the inlet fuel stream. The removal of H2S from fuel streams is expensive. Such costly fuel treatments to remove impurities as H2S prevent SOFC from competing with more traditional power generation methods. State-of-the-art sulfur tolerant anodes effectively react H2S, but show poor results when attempting to oxidize H2, making them inappropriate for power production.
H2S is typically removed during coal gasification by the Claus process, where a partial oxidation with air produces elemental sulfur and water. This process consists of two consecutive steps:2H2S+3O2→2SO2+2H2O  (4)2H2S+SO2→3S+2H2O  (5)The former reaction is carried out at temperatures nearing 1400 K as a non-catalytic combustion, while the latter reaction is a reversible catalytic process taking place over an equilibrium reactor train. The efficiency of this reaction scheme is limited by multiple side reactions, including the oxidation of sulfur:S+O2→SO2  (6)and a reverse Claus process:3S+H2OSO2+H2S  (7)
The largest contemporary obstacles to industrial or distributed use of SOFC are their susceptibility to poisoning by H2S impurities and the necessary costs of fuel treatment to remove H2S from syngas. While SOFC have shown encouraging stability and performance in systems containing only H2 and H2O, it is costly and difficult to locate and/or produce large quantities of pure elemental hydrogen. The damage to SOFCs if the H2S is not removed is unacceptable.
Due to the high operating temperature of the SOFC, H2S can also thermally decompose:H2S→½S2+H2  (8)The elemental sulfur and hydrogen produced by this chemical reaction may further react in the electrochemical reactionsH2+O2−→H2O+2e−  (9)½S2+2O2−→SO2+4e−  (10)where E0 for the reactions described by Equations (11) and (12) are 1.185 and 0.883 V, respectively. The simultaneous presence of H2S along with SO2 produced by the reaction described by Equation 12 at the SOFC anode may lead to their consumption via the reverse Claus process.
The contemporary standard for SOFC anodes is a metal such as Ni or Pt. These metals possess excellent catalytic activity toward H2 and CO oxidation at the temperatures (˜1000° C.) reached during SOFC operation. However, conventional SOFC anodes such as Ni or Pt are poisoned by H2S present in syngas, causing poor electrochemical performance and even irreversible system failure. For example, platinum catalyzes the oxidation of H2S to sulfur oxides at temperatures above 300° C. Researchers have examined the use of Pt as an anode in a SOFC utilizing an H2S-containing fuel stream, but Pt anodes have poor longevity when used with H2S-containing fuel streams due to the formation of PtS, which increases the interfacial resistance between the Pt anode and the YSZ electrolyte leading to detachment of Pt from YSZ. Fuel streams containing both 5% H2S (balance H2) and pure H2S have been tested, and it was found that longer anode lifetimes were achieved when using the dilute H2S feed.
Prior studies utilizing Pt as a SOFC anode in H2S-containing systems predominantly tested systems containing YSZ as the electrolyte. Such studies utilized ceria-based electrolytes in an effort to reduce the SOFC operating temperature. While low overpotentials and high current exchange densities were observed in such systems as in other Pt anode SOFC systems, the ceria electrolyte has been found to develop electronic conductivities in reducing environments and demonstrate poor long-term stability in a H2S environments. Corroborating the results of the previous researchers mentioned, the Pt anode demonstrated a steady loss in activity with time due to the formation of PtS.
Given that Pt anodes proved to be inappropriate for SOFC systems utilizing H2S-containing feeds, attention was given to contemporary Ni/YSZ anode SOFCs. A study using impedance analysis and DC polarization showed extensive sulfur poisoning due to the formation of NiS during operation. Since NiS has a melting point below the operating temperature of SOFCs, Ni-based anodes are susceptible to melting during operation with H2S-containing fuels. Differences in thermal expansion between Ni/YSZ and NiS can also prove problematic. Analogous to the results found for Pt anodes, it was found that the degree of sulfur poisoning on Ni/YSZ anodes is proportional to the total H2S content in the incoming fuel stream. Another study found that the polarization resistance for Ni/YSZ anodes doubled when a H2 fuel stream containing 5% H2S was utilized, while yet another study found that sulfur poisoning on Ni/YSZ anodes became irreversible after exposure to 105 ppm H2S at 1273 K.
Due to the infeasibility of Pt and Ni-based SOFC anodes in H2S environments, researchers have turned their attention to anodes made of perovskite oxides, which is a term for compounds having the generic composition ABO3. One study examined the properties of a wide range of perovskites based on lanthanum chromite (La1-xAxCr1-yByO3). While most of the materials tested somewhat fulfilled the requirements of an SOFC anode, none of the materials were found to have a combination of properties superior to Ni/YSZ. Poor conductivity, lacking activity toward hydrogen oxidation, and thermal expansions not matching those of YSZ or ceria-based electrolytes were among the disadvantages associated with using these materials as SOFC anodes.
Conventional studies of the properties of LaxSr1-xTiO3 (LST) found it to meet all requirements for SOFC anodes, and others successfully tested SOFCs utilizing LST anodes using fuel streams with concentrations of H2S ranging from 10 to 5000 ppm. These anodes showed little degradation over time and even showed an increase in activity when 5000 ppm H2S was present. This phenomenon was attributed to the SOFC oxidizing ˜12% of the available H2S, producing additional electricity.
LST anodes also oxidize other fuel gas species present in the fuel stream, such as H2, along with H2S, although the literature shows that LST does not have high electrocatalytic activity toward any fuel species. The overall electrocatalytic performance of LST anodes was noted in the literature to be far below that found using existing anode materials, such as Ni/YSZ. The maximum power density found using LST anodes is 175 mW/cm2, while power densities of up to 1.8 W/cm2 have been demonstrated by other contemporary systems.
More recent studies have shown that a perovskite known as lanthanum strontium vanadate (LaxSryVO3 or LSV) is not only resilient to H2S when used as a SOFC anode in 0-10% H2S environments, but further shows excellent activity toward H2S oxidation. LSV, however, does not show strong activity toward oxidation of other fuel gas species.
Studies using a Pt anode were also carried out for comparison. Performance of SOFCs utilizing LSV anodes showed no significant deterioration during a 48 hour period of operation in H2S environments. Moreover, the performance of the LSV anode appeared to increase as H2S concentration increased.